1. Field of the Invention
The present invention relates to a semiconductor device provided with a means of preventing a circuit from being damaged by unexpected high-voltage application such as electrostatic discharge.
2. Description of the Related Art
One of the main causes of the fault of integrated circuits is damage to a semiconductor element, an electrode, or the like due to electrostatic discharge (hereinafter referred to as “ESD”). In view of this, a protection circuit is provided between a terminal and an integrated circuit in order to prevent damage to the integrated circuit caused by ESD. A protection circuit is a circuit for preventing overvoltage or overcurrent called a surge or spike (hereinafter referred to as a surge), which occurs at the time of ESD phenomenon and exceeds the rating, from being supplied to an integrated circuit. Typical examples of an element used for a protection circuit include a resistor, a diode, and a capacitor.
For example, References 1 and 2 disclose that a diode is formed using a semiconductor layer formed over an insulating film and is used as an element for a protection circuit. In Reference 1, a lateral diode obtained by laterally forming a PN junction in a polysilicon film is provided between a high-frequency input-output signal line and an external power source VDD. In Reference 2, a PIN diode formed using a semiconductor layer is used as a protection element. Providing a floating electrode which faces an i-type layer of the PIN diode makes a p-type layer (or an n-type layer) of the PIN diode and the floating electrode short-circuited when a gate insulating film is damaged by overcurrent flowing through the protection circuit element and is electrically penetrated.